The pressure vessel in a boiler is usually made of steel (or alloy
steel), or historically of wrought iron. Stainless steel is
virtually prohibited (by the ASME Boiler Code) for use in wetted
parts of modern boilers, but is used often in superheater sections
that will not be exposed to liquid boiler water. In live steam
models, copper or brass is often used because it is more easily
fabricated in smaller size boilers. Historically, copper was often
used for fireboxes (particularly for steam locomotives), because of
its better formability and higher thermal conductivity; however, in
more recent times, the high price of copper often makes this an
uneconomic choice and cheaper substitutes (such as steel) are used
instead.

Steam boiler is a closed vessel in which water is heated. The
heated water exits the boiler for use in various process or heating
applications.